The present invention relates to apparatus and methods for an electric power alternate current (AC)-to-direct current (DC) converter employing composite technology and, more particularly, to apparatus and methods for an electric power AC-to-DC converter utilizing more than one type of conversion technology operating in parallel.
AC-to-DC converters play a significant role in the modern aerospace/military industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft.
The commercial aircraft business is moving toward MEA having no bleed-air environmental control systems (ECS), variable-frequency (VF) power distribution systems, and electrical actuation. A typical example is the Boeing 787 platform. The Airbus A350 airplane incorporates a large number of MEA elements. In the future, the next-generation Boeing airplane (replacement for the 737) and the Airbus airplane (replacement for the A320), will most likely use MEA. Some military aircraft already utilize MEA for primary and secondary flight control among other functions.
Military ground vehicles have migrated toward hybrid electric technology, where the main propulsion is performed by electric drives. Therefore, substantial demand for increased power electronics in that area has emerged. Future space vehicles will require electric power-generation systems for thrust vector and flight control actuation. These systems must be more robust and offer greatly reduced operating costs and safety compared to the existing Space Shuttle power systems.
These new aerospace and military trends have significantly increased electrical power-generation needs. The overall result has been a significant increase in the challenges to accommodate electrical equipment to the new platforms. A new set of electrical power quality and electromagnetic interference (EMI) requirements has been created to satisfy system quality and performance. The latest tendency, as a part of MEA, is the energy-efficient aircraft where electric power and heat management are inter-related. Therefore, overall system performance improvement and specifically power density increase are necessary for the new-generation hardware. This has led to increased operating voltages, and efforts to reduce system losses, weight, and volume. This particularly applies to the AC-to-DC conversion, which is a substantial contributor to the weight, volume, and cost of the power conversion electronics.
Power quality is a major concern for MEA aircraft because a large number of electric power systems and equipment are installed on the same bus. The power quality of these systems and equipment has much more stringent requirements to ensure that all power supplies/utilization equipment function properly together. For power supply equipment, additional monitoring features are implemented to detect and isolate equipment, or groups of equipment, that may experience a power quality issue. This isolation capability is to protect the other operating power supplies and utilization equipment.
For power utilization equipment, strict power quality requirements are imposed. Some reasons for the requirements are listed below:                Equipment contributing to power quality problems causes other equipment to fail.        Equipment is prevented from achieving its design performance or reliability due to the reduced power quality of the source.        Perhaps to meet a desired minimum weight, equipment designed with reduced or no power margin tends to be more susceptible to power quality issues. Also, equipment designed to minimize weight tends to create power quality issues.        Equipment can fail due to self-generated power quality problems.        
Power quality requirements for AC electrical equipment consist of a large number of parameters. Some of these are listed below:                Current distortion        Inrush current        Voltage distortion        Voltage modulation        Power factor        Phase balance        DC content        
Current distortions composed of AC harmonics are the key design drivers for equipment. The requirements for current harmonics, subharmonics, and interharmonics specify the allowable distortion as a function of multiples of the fundamental frequency of the input voltage. A typical current harmonic spectrum of an AC to DC converter includes all odd harmonics up to 39, with limits ranging from 10 to 0.25 percent of the maximum current fundamental. The current distortion requirement is a key design driver since it usually significantly impacts the equipment weight. Current distortion also is specified as a function of the equipment-rated power because the higher power equipment has more influence on the power bus.
For AC-to-DC converters, the requirements for the DC output are also important. The requirements include ripple voltage and voltage droop. The ripple voltage and voltage droop determine the DC operating range of the output equipment such as inverters.
When converting three-phase AC to DC, the most typical method is to employ a single three-phase full-wave rectifier in which six rectifying elements are connected in a bridge configuration. In such a three-phase full-wave rectifier, DC voltage is output by changing over the rectifying elements so that they successively conduct at intervals of 60°. However, with this method, the rectified DC voltage contains a voltage ripple of large amplitude having a period of six times the power source frequency, producing harmonics.
As can be seen, there is a need for an improved AC-to-DC converter that may provide sufficient power density while not substantially adding to the weight, volume and cost of the power conversion electronics.